In conventional pay TV broadcast systems, the payload of every packet is scrambled. The key required for descrambling is provided only to authorized subscribers, in order to prevent unauthorized parties from viewing the broadcast content.
Methods have been developed for encrypting only a portion of the video data stream. For example, U.S. Patent Application Publication 2007/0083467, whose disclosure is incorporated herein by reference, describes techniques for partially encrypting media files. Although only partial encryption is used, the portion being encrypted disrupts the usefulness of other, non-encrypted parts of the media file. The disclosed techniques are said to render the media file substantially unusable by unauthorized users. Since the media files are only partially encrypted, the decryption is likewise only partial, which makes decryption by authorized client devices faster and less resource-intensive.
As another example, U.S. Pat. No. 7,124,303, whose disclosure is incorporated herein by reference, describes an encryption arrangement for television programs in which only a portion of the data required for full presentation of a program is encrypted. In one embodiment, only audio data are encrypted, leaving video in the clear. This sort of approach is said to permit coexistence of multiple conditional access encryption systems, with multiple manufacturers' set-top boxes, within a single system.